I recently moved to Rhode Island and my mash efficiency has suffered. The main part of my process that has changed is the water. The pH as measured by colorpHast strips (and two other off brands) starts out around 5.5 (with the systematic error of 0.3 as observed by Braukaiser, that's 5.8).

This pH is ridiculously low to work with, and I've been having a hard time. I've used more calcium carbonate (chalk) and sodium bicarbonate (baking soda) than I like to raise the pH to acceptable levels in the mash (it gets to the 4-5 pH range even with a mash of around 14 SRM).

I've also taken to steeping roasted (and really any 100+ Lovibond grain) grains for pretty much every beer I mash to avoid dropping the pH with darker grains.

What else can I use to raise the pH?

I've heard sodium hydroxide (food grade) might be usable, but I have no experience with it and haven't found much about it with regard to mashing wort. It's also caustic and somewhat dangerous to handle from what I hear.

I've also heard that calcium hydroxide (used for pickling) is an option. I think I might order some of this from Amazon and try it out.

Your water is pretty soft! That puts you in a similar position to brewers that build up their water profile from distilled/RO water. Adding chalk/baking soda is the usual way to go. Any reason why you can't do that? Sure, chalk is a pain to dissolve and baking soda contains sodium, but you're only adding it to approximate established profiles that are known to work - you'll probably end up much less than than 100ppm sodium, which adds body, and is far below the 200ppm that's considered to affect taste.
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mdma♦Mar 2 '12 at 14:38

I do use both chalk and baking soda to begin with, but it changes the flavor profile enough that I try not to put too much in (and I can get to "too much" in even brown beers). I've heard Calcium Hydroxide is actually probably better than Sodium Hydroxide. Hrm...
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Hop the Mad AlchemistMar 2 '12 at 16:26

Sodium hydroxide is lye - caustic soda, and has the same disadvantage as baking soda - leaving behind sodium anions, as well as being a strong base, so careful handling is required. Calcium hydroxide - pickling lye - is better suited, it's also a strong base and is used much in the food industry. (I live in Norway, and here it's used to prepare a fish meal eaten at Christmas.) I don't like that dish so I've not used it myself, but it's a powder that's fairly easy to handle. Weigh out half a gram, 1/8tsp and observe the change in pH, and add more as necessary. It produces Calcium anions - adding calcium is beneficial since all beers need at least 50ppm calcium.

If you prefer to avoid handling strong bases, Chalk is ok for the beer, since it avoids sodium, but doesn't readily dissolve. (Although with your acidic mash you might get better results than most.) A trick to getting it to dissolve is to emulate what mother nature does - constant motion (flowing waters) and dissolved CO2 (from the air) help dissolve the chalk into solution. You can speed up the natural process by adding the water and chalk to a corny keg, adding a little CO2 pressure and shaking/rocking.